The invention is useful for high speed electrolytic in-process dressing (HELID) or sharpening of grinding wheels, especially diamond or CBN wheels. Grinding is the dominant machining process to achieve high precision and is widely used in various industries to produce precision metal and ceramic parts. The device and process of the present invention is useful for sharpening of fixed abrasive tools without stop and slow down of a machining process. The device is compact, low-cost and user friendly.
The role of grinding processes in industry is becoming more and more important due to the increasing need for cost-effective machining of semiconductor materials with nano-precision such as super large and super-flat silicon wafers (Abe et al. Proceedings of JSPE 1998 Spring Conference 1998.471-472), and the high-speed machining of hard-to-machine materials including advanced ceramics, super-alloys, and composites (Kovach, et al. ONRL/TM-13562 1997.102-107). Usually carried out at around 30 m/s, grinding processes have been pushed toward nano-precision and high-speed ranging from 100 to 350 m/s to increase the productivity and quality of industrial products cost-effectively (Salmon, World Scientific 1997.126-133; Inasaki, Annals of the CIRP 1993.42(2) :723-731; Komanduri, Annals of the CIRP 1997.46(2):97). The field of grinding has expanded from classical finishing-machining to highly efficient machining in Japan, Europe and the USA (Kloke et al. Annals of the CIRP 1997. 46(2):715-723).
Traditionally, grinding wheels have been consumed in the grinding process usually by being ground or cut away by wheel sharpening dressers. As much as 90% of the grinding wheel materials can be lost during dressing, leaving only 10% of the wheel materials to be used in grinding (Kovacevic, Abrasives, 1997.June/July:10-25). Most of the grinding energy is consumed in rubbing the surface of a work piece by a dull grinding wheel, instead of cutting the surface clearly (Malkin, Ellis Horwood Limited, 1989; Salmon, Modern Grinding Process Technology, McGraw Hill, 1992). Wheel consumption accounts for about 60% of the grinding cost of steel materials using CBN wheels (Westkamper and Tonshoff, Annals of the CIRP 1993. 42(1):371-374). As reported by NIST, the grinding cost of ceramic materials may reach up to 75% of the total component cost mainly due to excessive wheel consumption and excessive time spent on grinding the hard-to machine-materials (Jahanmir et al. NIST Special Publication 1992.834).
The majority of grinding wheels are being dressed with conventional dressers including single-point diamond, multi-point diamond, crush roll and diamond roll. Abrasive dressing sticks are also used. For many grinding machines, dressing may be time consuming due to the need to stop the grinding process or slow the wheel down to a required speed and slowly feeding the dresser.
In-process dressing can be carried out by equipping the grinding machines with accurate and expensive in-process dressing devices. However, inconsistent dressing and an unstable layer on grinding wheel surfaces are still serious problems to overcome. The wear of a dresser and the skill of an operator are also factors causing inconsistent dressing. As a result, inconsistent surface finish, and form and size inaccuracies are commonly found on ground workpieces. Traditional dressing and grinding processes are regarded as temperamental and depend greatly on operator skills. Methods have been developed for automatic and consistent sharpening of grinding wheels. ELID or electrolytic in-process dressing method is one of the latest promising dressing methods (Ohmori and Nakagawa, Annals of the CIRP 1990.39(1)(90):329-332). An ELID system consists of an electric conductive cast-iron fiber bonded (CIFB) grinding wheel as an anode, a copper or graphite cathode, and a power unit. When the wheel is subjected to a weak DC pulse current in an aqueous alkaline electrolyte, rusting of the wheel surface is promoted. The strong cast-iron bond will be turned into rather soft oxides and form a layer with poor electric conductivity. As the layer forms on the wheel surface, the current will become smaller, consequently, electrolysis of the iron bond will be suppressed to a minimum. As the grinding proceeds, chips of the materials being ground dispense the layer and make it thinner. Then, ELID current flow will resume. Subsequent increase in ELID current will attack the iron bond, turn it into the oxide layer and leave new protrusions of the diamond grains. The process continues during the whole period of ELID grinding, regardless of the grain size. Over the past ten years, ELID grinding has been studied intensively in Japan (Ohmori and Nakagawa, Annals of the CIRP 1990.39(1) (90) :329-332; Ohmori et al. Annals of the CIRP 1995.44(1):287-290; Ohmori and Nakagawa, Annals of the CIRP 1997.46(1):261-264; Suzuki, Annals of the CIRP 1991.40(1):363-366; Enomoto and Shimazaki, U.S. Pat. No. 5,868,607). The consistency and efficiency of ELID grinding have been recognized internationally, (Inasaki et al. Annals of the CIRP 1993.42(2):723-731; Salmon Advances in Abrasive Technology, World Scientific 1997.126-133; Lee and Kim, Int. J. Mach. Tools Manufact. 1997.37(12) :1673-1689; Bandyopahyay, Abrasives 1997.April/May:10-34; Bandyopahyay, ONRL/SUB/96-SV16/1 1997.1-65; Zhang et al. xe2x80x9cGrinding of GS-44 Silicon Nitride Using Both Vitrified and CIFB Diamond Wheels, Cost-Effective Ceramic Grinding: The Effect of Machine Stiffness on the Grinding of Silicon Nitridexe2x80x9d DE-AC05-96OR22464:SU366-19, 1996; Bifano et al. Manufacturing Science and Manufacturing 1995., Med-Vol. 2-1/MH vol. 3-1:329-348). However, the consistency and efficiency of ELID grinding are only realized at a low surface speed of about 20 m/s and no higher than 30 m/s. The dressing efficiency drops when the wheel surface speeds are larger than the effective surface speeds. Therefore, a dull grinding wheel can no longer be sharpened and efficient grinding cannot be realized. ELID systems also prove to be ineffective due to decreasing dressing current, with wheel speed increases. Such a low dressing current indicates a high resistance due to insufficient electrolyte in the dressing zone. The insufficient electrolyte along the dressing zone is caused by air film surrounding the wheel, voids behind protrusions, leaking of fluid in transverse direction, and centrifugal force as the wheel speed increases.
The present invention provides a new device for realizing electrolytic in-process dressing of grinding wheels at high grinding surface speeds with quasi-static foil electrode and film terminals. The electrolytic in-process dressing grinding has never before been realized at high speeds. The instant invention also provides a high-speed electrolytic in-process dressing (HELID) method for sharpening superabrasive grinding wheels consistently using electrolytic in-process dressing to realize high-speed ultra-precision grinding.
A device for high speed electrolytic in-process dressing (HELID) comprising an electrical conductive foil electrode, an electrical conductive bond grinding wheel, an electrolytic fluid supply and a power source is provided. Also provided is a method for using the HELID for sharpening grinding wheels comprising rotating a grinding wheel at a desired speed while supplying electrolytic fluid between the wheel surface and a foil electrode thereby allowing the foil to wrap around the grinding wheel by way of hydrodynamic forces to form a thin hydrodynamic film bearing between the wheel surface and the traveling foil.